NIH/3T3 fibroblasts, Kirsten sarcoma virus (Ki-MuSV) transformed NIH/3T3 cells, and cellular revertants of these cells which are resistant to transformation by specific oncogenes have been utilized to determine possible cellular components involved in the malignant tranformation of cells. A solid phase radioimmunoassay was developed to measure the levels of 53 K cellular protein which is elevated in several types of malignant cells. NIH/3T3 cells were found to have elevated levels of p53 protein relative to other normal cell types, and transformation of these cells by Ki-MuSV caused a 2-to-5 fold increase in p53. The revertant cells, which are resistant to transformation by ras p21 oncogene product, exhibit levels of p53 protein only 1/3 that of the NIH/3T3 cells. Studies indicate that p53 protein is elevated in normal cells within 3 to 6 hours after treatment with phorbol ester tumor promoter. Calcium activated, phospholipid-dependent protein kinase (PK-C) appears to be involved in regulating cell growth. This kinase serves as the cellular phorbol ester receptor to mediate early events of tumor promotion. PK-C activity is found to be elevated in the particulate fraction of cells under conditions of phorbol ester tumor promotion and low population density, rapid cell growth. Ki-MuSV transformed NIH/3T3 cells also have an increased amount of PK-C activity in the particulate fraction when compared to control, growing NIH/3T3 cells. The revertant cells exhibit low membrane-associated PK-C activity when compared to the parent NIH/3T3 cells. Thus, transformation-induced and phorbol ester-induced changes in p53 protein and in association of PK-C with the plasma membrane may be critical events in mediating eventual malignant transformation.